Recently we have extended our comparative studies on renal handling of folic acid, folinic acid and methotrexate in the monkey. Stop-flow experiments have demonstrated that the (U/P folic)/(U/P IN) ratio during the free flow periods was below one, but increased to a peak value 2-fold above baseline in stop-flow samples derived from the proximal tubules, indicating a net secretion during the stop-flow period. Folic acid secretion was inhibited by PAH and Ns salicylate. The stop-flow pattern of MTX excretion was similar, except that the baseline (U/P MTX)/(U/P IN) ratio was greater than one, indicating net secretion with no or minimum reabsorption during both free and stopped urinary flow. MTX secretion was inhibited by PAH but not by folic, folinic acid or glutamic acid. The mechanisms of the different renal handling of folic acid and MTX is still unclear, is continuously investigated in this laboratory. Furthermore, we plan to study the interaction between the renal excretion of MTX and of several organic acids. The first question to be answered is whether the renal secretion of MTX can be reduced by the administration of the tested organic acid, thereby in decreasing nephrotoxicity induced by MTX. Secondly, our laboratory is also engaged in investigating the effect of ions such as the Na ion, on renal secretion of MTX. Using the micro-infusion technique we are studying the secretory rate or reabsorptive rate of MTX vs. Na ion.